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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="the_type_traits_introspection_library.tti_functionality"></a><a class="link" href="tti_functionality.html" title="General Functionality">General
    Functionality</a>
</h2></div></div></div>
<div class="toc"><dl class="toc"><dt><span class="section"><a href="tti_functionality.html#the_type_traits_introspection_library.tti_functionality.tti_functionality_nm_gen">Macro
      metafunction name generation considerations</a></span></dt></dl></div>
<p>
      The elements of a type about which a template metaprogrammer might be interested
      in finding out at compile time are:
    </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
          Does it have a nested type with a particular name ?
        </li>
<li class="listitem">
          Does it have a nested type with a particular name which fulfills some other
          possibility for that nested type.
        </li>
<li class="listitem">
          Does it have a particular user-defined nested type (struct/class, union,
          or enumaeration) with a particular name ?
        </li>
<li class="listitem">
          Does it have a particular user-defined nested type with a particular name
          which fulfills some other possibility for that nested type.
        </li>
<li class="listitem">
          Does it have a nested class template with a particular name ?
        </li>
<li class="listitem">
          Does it have a nested class template with a particular name and a particular
          signature ?
        </li>
<li class="listitem">
          Does it have a nested member function template with a particular name and
          a particular instantiated signature ?
        </li>
<li class="listitem">
          Does it have a nested static function template with a particular name and
          a particular instantiated signature ?
        </li>
<li class="listitem">
          Does it have a member function with a particular name and a particular
          signature ?
        </li>
<li class="listitem">
          Does it have member data with a particular name and of a particular type
          ?
        </li>
<li class="listitem">
          Does it have a static member function with a particular name and a particular
          signature ?
        </li>
<li class="listitem">
          Does it have static member data with a particular name and of a particular
          type ?
        </li>
<li class="listitem">
          Does it have either member data or static member data with a particular
          name and of a particular type ?
        </li>
<li class="listitem">
          Does it have either a member function or static member function with a
          particular name and of a particular type ?
        </li>
</ul></div>
<p>
      These are some of the compile-time questions which the TTI library answers.
      It does this by creating metafunctions, which can be used at compile-time,
      using C++ macros. Each of the metafunctions created returns a compile time
      constant bool value which answers one of the above questions at compile time.
      When the particular element above exists the value is 'true', or more precisely
      boost::mpl::true_, while if the element does not exist the value is 'false',
      or more precisely boost::mpl::false_. In either case the type of this value
      is boost::mpl::bool_.
    </p>
<p>
      This constant bool value, in the terminology of the Boost MPL library, is called
      an 'integral constant wrapper' and the metafunction generated is called a 'numerical
      metafunction'. The results from calling the metafunction can be passed to other
      metafunctions for type selection, the most popular of these being the boolean-valued
      operators in the Boost MPL library.
    </p>
<p>
      All of the questions above attempt to find an answer about an inner element
      with a particular name. In order to do this using template metaprogramming,
      macros are used so that the name of the inner element can be passed to the
      macro. The macro will then generate an appropriate metafunction, which the
      template metaprogrammer can then use to introspect the information that is
      needed. The name itself of the inner element is always passed to the macro
      as a macro parameter, but other macro parameters may also be needed in some
      cases.
    </p>
<p>
      All of the macros start with the prefix <code class="computeroutput"><span class="identifier">BOOST_TTI_</span></code>,
      create their metafunctions as class templates in whatever scope the user invokes
      the macro, and come in two forms:
    </p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
          In the simplest macro form, which I call the simple macro form, the 'name'
          of the inner element is used directly to generate the name of the metafunction
          as well as serving as the 'name' to introspect. In generating the name
          of the metafunction from the macro name, the <code class="computeroutput"><span class="identifier">BOOST_TTI_</span></code>
          prefix is removed, the rest of the macro name is changed to lower case,
          and an underscore ( '_' ) followed by the 'name' is appended. As an example,
          for the macro <code class="computeroutput"><span class="identifier">BOOST_TTI_HAS_TYPE</span><span class="special">(</span><span class="identifier">MyType</span><span class="special">)</span></code> the name of the metafunction is <code class="computeroutput"><span class="identifier">has_type_MyType</span></code> and it will look for
          an inner type called 'MyType'.
        </li>
<li class="listitem">
          In a more complicated macro form, which I call the complex macro form,
          the macro starts with <code class="computeroutput"><span class="identifier">BOOST_TTI_TRAIT_</span></code>
          and a 'trait' name is passed as the first parameter, with the 'name' of
          the inner element as the second parameter. The 'trait' name serves solely
          to completely name the metafunction in whatever scope the macro is invoked.
          As an example, for the macro <code class="computeroutput"><span class="identifier">BOOST_TTI_TRAIT_HAS_TYPE</span><span class="special">(</span><span class="identifier">MyTrait</span><span class="special">,</span><span class="identifier">MyType</span><span class="special">)</span></code> the name of the metafunction is <code class="computeroutput"><span class="identifier">MyTrait</span></code> and it will look for an inner
          type called <code class="computeroutput"><span class="identifier">MyType</span></code>.
        </li>
</ol></div>
<p>
      Every macro metafunction has a simple macro form and a corresponding complex
      macro form. Once either of these two macro forms are used for a particular
      type of inner element, the corresponding macro metafunction works exactly the
      same way and has the exact same functionality.
    </p>
<p>
      In the succeeding documentation all macro metafunctions will be referred by
      their simple form name, but remember that the complex form can always be used
      instead. The complex form is useful whenever using the simple form could create
      a duplicate name in the same name space, thereby violating the C++ one definition
      rule.
    </p>
<h4>
<a name="the_type_traits_introspection_library.tti_functionality.h0"></a>
      <span class="phrase"><a name="the_type_traits_introspection_library.tti_functionality.macro_metafunction_name_generation"></a></span><a class="link" href="tti_functionality.html#the_type_traits_introspection_library.tti_functionality.macro_metafunction_name_generation">Macro
      Metafunction Name Generation</a>
    </h4>
<p>
      For the simple macro form, even though it is fairly easy to remember the algorithm
      by which the generated metafunction is named, TTI also provides, for each macro
      metafunction, a corresponding 'naming' macro which the end-user can use and
      whose sole purpose is to expand to the metafunction name. The naming macro
      for each macro metafunction has the form: 'corresponding-macro'_GEN(name).
    </p>
<p>
      As an example, BOOST_TTI_HAS_TYPE(MyType) creates a metafunction which looks
      for a nested type called 'MyType' within some enclosing type. The name of the
      metafunction generated, given our rule above is 'has_type_MyType'. A corresponding
      macro called BOOST_TTI_HAS_TYPE_GEN, invoked as BOOST_TTI_HAS_TYPE_GEN(MyType)
      in our example, expands to the same 'has_type_MyType' name. These name generating
      macros, for each of the metafunction generating macros, are purely a convenience
      for end-users who find using them easier than remembering the name-generating
      rule given above.
    </p>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="the_type_traits_introspection_library.tti_functionality.tti_functionality_nm_gen"></a><a class="link" href="tti_functionality.html#the_type_traits_introspection_library.tti_functionality.tti_functionality_nm_gen" title="Macro metafunction name generation considerations">Macro
      metafunction name generation considerations</a>
</h3></div></div></div>
<p>
        Because having a double underscore ( __ ) in a name is reserved by the C++
        implementation, creating C++ identifiers with double underscores should be
        avoided by the end-user. When using a TTI macro to generate a metafunction
        using the simple macro form, TTI appends a single underscore to the macro
        name preceding the name of the element that is being introspected. The reason
        for doing this is because Boost discourages as non-portable C++ identifiers
        with mixed case letters and the underscore then becomes the normal way to
        separate parts of an identifier name so that it looks understandable. Because
        of this decision to use the underscore to generate the metafunction name
        from the macro name, any inner element starting with an underscore will cause
        the identifier for the metafunction name being generated to contain a double
        underscore.
      </p>
<p>
        A rule to avoid this problem is:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
            When the name of the inner element to be introspected begins with an
            underscore, use the complex macro form, where the name of the metafunction
            is specifically given.
          </li></ul></div>
<p>
        Furthermore because TTI often generates not only a metafunction for the end-user
        to use but some supporting detail metafunctions whose identifier, for reasons
        of programming clarity, is the same as the metafunction with further letters
        appended to it separated by an underscore, another rule is:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
            When using the complex macro form, which fully gives the name of the
            generated macro metafunction, that name should not end with an underscore.
          </li></ul></div>
<p>
        Following these two simple rules will avoid names with double underscores
        being generated by TTI.
      </p>
<h5>
<a name="the_type_traits_introspection_library.tti_functionality.tti_functionality_nm_gen.h0"></a>
        <span class="phrase"><a name="the_type_traits_introspection_library.tti_functionality.tti_functionality_nm_gen.reusing_the_named_metafunction"></a></span><a class="link" href="tti_functionality.html#the_type_traits_introspection_library.tti_functionality.tti_functionality_nm_gen.reusing_the_named_metafunction">Reusing
        the named metafunction</a>
      </h5>
<p>
        When the end-user uses the TTI macros to generate a metafunction for introspecting
        an inner element of a particular type, that metafunction can be re-used with
        any combination of valid template parameters which involve the same type
        of inner element of a particular name.
      </p>
<p>
        As one example of this let's consider two different user-defined types called
        'AType' and 'BType', for each of which we want to determine whether an inner
        type called 'InnerType' exists. For both these types we need only generate
        a single macro metafunction in the current scope by using:
      </p>
<pre class="programlisting"><span class="identifier">BOOST_TTI_HAS_TYPE</span><span class="special">(</span><span class="identifier">InnerType</span><span class="special">)</span>
</pre>
<p>
        We now have a metafunction, which is a C++ class template, in the current
        scope whose C++ identifier is 'has_type_InnerType'. We can use this same
        metafunction to introspect the existence of the nested type 'InnerType' in
        either 'AType' or 'BType' at compile time. Although the syntax for doing
        this has no yet been explained, I will give it here so that you can see how
        'has_type_InnerType' is reused:
      </p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
            'has_type_InnerType&lt;AType&gt;::value' is a compile time constant telling
            us whether 'InnerType' is a type which is nested within 'AType'.
          </li>
<li class="listitem">
            'has_type_InnerType&lt;BType&gt;::value' is a compile time constant telling
            us whether 'InnerType' is a type which is nested within 'BType'.
          </li>
</ol></div>
<p>
        As another example of metafunction reuse let's consider a single user-defined
        type, called 'CType', for which we want to determine if it has either of
        two overloaded member functions with the same name of 'AMemberFunction' but
        with the different function signatures of 'int (int)' and 'double (long)'.
        For both these member functions we need only generate a single macro metafunction
        in the current scope by using:
      </p>
<pre class="programlisting"><span class="identifier">BOOST_TTI_HAS_MEMBER_FUNCTION</span><span class="special">(</span><span class="identifier">AMemberFunction</span><span class="special">)</span>
</pre>
<p>
        We now have a metafunction, which is a C++ class template, in the current
        scope whose C++ identifier is 'has_member_function_AMemberFunction'. We can
        use this same metafunction to introspect the existence of the member function
        'AMemberFunction' with either the function signature of 'int (int)' or 'double
        (long)' in 'CType' at compile time. Although the syntax for doing this has
        no yet been explained, I will give it here so that you can see how 'has_type_InnerType'
        is reused:
      </p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
            'has_member_function_AMemberFunction&lt;CType,int,boost::mpl::vector&lt;int&gt;
            &gt;::value' is a compile time constant telling us whether 'AMemberFunction'
            is a member function of type 'CType' whose function signature is 'int
            (int)'.
          </li>
<li class="listitem">
            'has_member_function_AMemberFunction&lt;CType,double,boost::mpl::vector&lt;long&gt;
            &gt;::value' is a compile time constant telling us whether 'AMemberFunction'
            is a member function of type 'CType' whose function signature is 'double
            (long)'.
          </li>
</ol></div>
<p>
        These are just two examples of the ways a particular macro metafunction can
        be reused. The two 'constants' when generating a macro metafunction are the
        'name' and 'type of inner element'. Once the macro metafunction for a particular
        name and inner element type has been generated, it can be reused for introspecting
        the inner element(s) of any enclosing type which correspond to that name
        and inner element type.
      </p>
<h5>
<a name="the_type_traits_introspection_library.tti_functionality.tti_functionality_nm_gen.h1"></a>
        <span class="phrase"><a name="the_type_traits_introspection_library.tti_functionality.tti_functionality_nm_gen.avoiding_odr_violations"></a></span><a class="link" href="tti_functionality.html#the_type_traits_introspection_library.tti_functionality.tti_functionality_nm_gen.avoiding_odr_violations">Avoiding
        ODR violations</a>
      </h5>
<p>
        The TTI macro metafunctions are generating directly in the enclosing scope
        in which the corresponding macro is invoked. This can be any C++ scope in
        which a class template can be specified. Within this enclosing scope the
        name of the metafunction being generated must be unique or else a C++ ODR
        ( One Definition Rule ) violation will occur. This is extremely important
        to remember, especially when the enclosing scope can occur in more than one
        translation unit, which is the case for namespaces and the global scope.
      </p>
<p>
        Because of ODR, and the way that the simple macro form metafunction name
        is directly dependent on the inner element and name of the element being
        introspected, it is the responsibility of the programmer, using the TTI macros
        to generate metafunctions, to avoid ODR within a module ( application or
        library ). There are a few general methods for doing this:
      </p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
            Create unique namespace names in which to generate the macro metafunctions
            and/or generate the macro metafunctions in C++ scopes which can not extend
            across translation units. The most obvious example of this latter is
            within C++ classes.
          </li>
<li class="listitem">
            Use the complex macro form to specifically name the metafunction generated
            in order to provide a unique name.
          </li>
<li class="listitem">
            Avoid using the TTI macros in the global scope.
          </li>
</ol></div>
<p>
        For anyone using TTI in a library which others will eventually use, it is
        important to generate metafunction names which are unique to that library.
      </p>
<p>
        TTI also reserves not only the name generated by the macro metafunction for
        its use but also any C++ identifier sequence which begins with that name.
        In other words if the metafunction being generated by TTI is named 'MyMetafunction'
        using the complex macro form, do not create any C++ construct with an identifier
        starting with 'MyMetaFunction', such as 'MyMetaFunction_Enumeration' or 'MyMetaFunctionHelper'
        in the same scope. All names starting with the metafunction name in the current
        scope should be considered out of bounds for the programmer using TTI.
      </p>
</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2011-2013 Tropic Software
      East Inc<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
</div></td>
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